Fast advances in the development of Yb-doped fiber lasers have changed the landscape of high-power lasers during the last decade. Yb-doped fiber lasers uniquely combine the diffraction-limited beam quality, distributed thermal loading and compact and modular packaging of fiber systems with high electrical-to-optical efficiency and broad gain bandwidth of Yb-gain medium that translates to high-average-power compact systems over a wide wavelength range. The fiber lasers have demonstrated up to a few thousand Watts (kW) with broad linewidths but the fiber laser output power is not expected to scale much beyond 10 kW due to thermal loading and optical nonlinear effects.
To scale the output power of fiber-laser systems further, beam-combination approaches have been proposed and they are broadly classified into coherent-beam combination and spectral-beam combination. Both approaches require narrow linewidths on the order of a few GHz, which makes the power scaling of individual fiber laser channels challenging, primarily due to limitation posed by stimulated Brillouin scattering (SBS) in fiber. (Stimulated Brillouin scattering (SBS) is a well-known phenomenon that can lead to power limitations or even the destruction of a high-power fiber-laser system due to sporadic or unstable feedback, self-lasing, pulse compression and/or signal amplification.) Innovations in fiber and component technologies and fiber-laser designs have pushed the power to a few hundred Watts at narrow linewidths and expected to push the power to a few kW in the coming years. Beam-combination approaches can potentially combine multiple fiber-laser channels and scale the output power of fiber laser systems over 100 kW.
Coherent-beam combination involves vectorially summing the output from multiple lasers by phase locking the individual emitters to a single frequency (1: T. M. Shay, V. Benham, J. T. Baker et al., “First experimental demonstration of self-synchronous phase locking of an optical array”, Opt. Exp., 14, 12022-12027 (2006)). Coherent-beam combination produces spectrally bright beams but suffers from multi-lobed far-field transverse profiles with off-axis sidelobes. Approaches to reduce the sidelobe power in coherent combining have also been explored (2: S. Christensen, “Novel coherent beam combiner,” presented at the Solid State Diode Laser Technol. Rev., Albuquerque, N. Mex., Jun. 13-15, (2006), Paper BC-4; and 3: T. Y. Fan, “Laser beam combining for high-power, high-radiance sources”, IEEE J. Quantum Electron., vol. 11, 567-577 (2005)).
Spectral-beam combination (SBC) circumvents the problem of sidelobe power in transverse-field profiles by trading spectral brightness for spatial brightness (4: E. J. Bochove, “Theory of spectral beam combining of fiber lasers,” IEEE J. Quantum Eletron., 38, 432-445 (2002); and 5: S. J. Augst, A. K. Goyal, R. L. Aggarwal, T. Y. Fan and A. Sanchez, “Wavelength beam combining of ytterbium fiber lasers,” Opt. Lett., 28, 331-333 (2003)). In SBC, a diffraction grating is used to merge spectrally distinct output from multiple fiber lasers to a spatially bright diffraction-limited beam. Lockheed Martin Aculight Corporation has demonstrated over 500 W of output using SBC with efficiency and beam quality rivaling that of the individual fiber-laser output. Several SBC techniques have been demonstrated at Lockheed Martin Aculight Corporation and in this application some of the experimental results obtained at Lockheed Martin Aculight Corporation are described.
The present invention describes improvements and builds upon important SBC ring-laser ideas and designs that were co-developed by Eric C. Honea, Thomas H. Loftus and Bernard G. Deuto.
With a simple optical design, it is possible to construct a compact SBC system that operates with a large number of emitters to produce a collimated output beam with the combined wavelengths. FIG. 1 is a schematic illustration of a linear-oscillator SBC system 100 where a partial reflector 140 provides feedback to each gain element 110 at the wavelength needed to provide a single output beam 69, where some of the beam 68 is reflected back to the grating 130, and focussing element 120 into gain elements 110 originally reported by Daneu et al. for an array of diode emitters (6: V. Daneu, A. Sanchez, T. Y. Fan, H. K. Choi, G. W. Turner, and C. C. Cook, “Spectrally beam combining of broad stripe diode laser array in an external cavity,” Opt. Lett., vol. 25, pp. 405-407 (2000)). System 100 has the disadvantage that the highest-power beam (output beam 69) passes through the output reflector 140, which can result in undesirable energy absorption in the output reflector 140. System 100 also has the disadvantage that output beam 69 continues to chromatically disperse after it diffracts from uncompensated grating 130 and passes through the output reflector 140, which can result in undesirable beam quality. The grating equation defines the wavelengths in the system according to:
                              Δ          ⁢                                          ⁢          λ                =                                            (                              Δ                ⁢                                                                  ⁢                x                            )                        ⁢            d            ⁢                                                  ⁢                          Cos              ⁡                              (                                  θ                  g                                )                                              f                                    (        1        )            Here, Δx is the spacing between laser emitters 110, d is the spacing of the grating line grooves of grating 130, θg is the grating diffraction angle, f the focal length of the collimation lens/mirror 120, and Δλ the wavelength difference between emitters 110 in order to produce a single collimated output beam 69. The focal length typically defines the longest dimension in the optical system 100, with hundreds of elements easily combined in a compact optical system. For instance, with a 1,600-line/mm grating, a focal length of 40 cm, a grating angle of 58 degrees and a wavelength spread of 1040-1060 nm, one obtains an array width of ˜2.5 cm. With a fiber spacing of 250 microns, this corresponds to approximately one-hundred (100) gain elements. Tighter element spacing, or a longer focal length, enables the combination of larger numbers of elements.
The linear-oscillator approach has been applied to both diode-laser and fiber-laser arrays. The design has been applied to a number of diode array configurations, including an array of 200 single-mode lasers within a single diode-laser bar (7: S. C. Tidwell et al, “Spectral beam combining of diode-laser bars achieve efficient near diffraction limited output power,” Proc. SPIE 4973-08 (2003)) and an array of 1,400 single-mode lasers from seven diode-laser bars (8: C. Hamilton, S. Tidwell, D. Meekhof, J. Seamans, N. Gitkind and D. Lowenthal, “Spectral beam combining of a broad-stripe diode laser array in an external cavity,” Proc. SPIE, 5336-1 (2004)).
One of the challenges in early applications of the optical design of FIG. 1 to fiber lasers was the operation of the fiber lasers as linear oscillators with the required narrow linewidth. Narrow-linewidth operation of fiber linear oscillators has shown limited power scaling (9: A. Liu, R. Mead, T. Vatter, A. Henderson and R. Stafford, “Spectral beam combining of high power fiber lasers,” Proceedings of SPIE 5335, 81-88 (2004)). High-power, narrow-line-width operation has been demonstrated in master-oscillator power-amplifier (MOPA) fiber-laser configurations. Scientists at Lockheed Martin Aculight Corporation have developed 200-300-W MOPA fiber lasers with the required linewidth and polarized output for SBC. Using these MOPA lasers, we have demonstrated SBC of two- and three-fiber lasers (10: T. Loftus et al., “Spectrally Beam-Combined Fiber Lasers for High-Average-Power Applications” IEEE Journal of Selected Topics in Quantum Electronics, Volume 13, Issue 3, 487-497 (2007)) with over 500 Watts and near-diffraction-limited output.
FIG. 2A is a schematic illustration of a prior-art spectral-beam combination of two fiber laser channels (beams 71 and 72 from fiber lasers 210, where the fiber ends are held by a spatial array 212) using a transform lens 220 (or a focusing reflector, not shown, used in its place to perform a corresponding focusing function) and a single output grating 230. The resulting output beam 79 in FIG. 2A has the advantage of not passing through an output reflector 140, as was the case for output beam 69 of FIG. 1, but still has the disadvantage of chromatic dispersion introduced by the output grating 230. FIG. 2B is a schematic diagram of a Gaussian beam with a finite spectral linewidth Δλ, diffracted by a grating. In some embodiments, unlike the configuration in FIG. 1, master-oscillator power-amplifier (MOPA) lasers are used wherein the wavelength and linewidth of each channel are set with an external seed source (e.g., a master oscillator, which feeds a seed signal into a power amplifier that outputs one of the laser beams 71-72 in the MOPA SBC configuration), rather than wavelength-dispersed optical feedback from a partially reflecting mirror, such as mirror 140 of FIG. 1, and grating 130. One can analyze the linewidth requirements for the fiber MOPA systems by considering the dependence of the combined beam quality on the single-channel linewidth for a simple single-grating SBC system, consisting of a fiber array, a transform optic, and a single diffraction grating to combine the beam (see FIG. 2A).
Consider a single-mode Gaussian beam with a linewidth Δλ, incident on a diffraction grating as shown in FIG. 2B. For simplicity, one can assume the Rayleigh range for the beam is much longer than other length scales of interest (this is true for beam diameters larger than a few mm). The beam quality for the diffracted beam is then given by
                    BQ        =                                            ω              1                        ⁢                          θ              1                                                          ω              0                        ⁢                          θ              0                                                          (        2        )            where ω0 and θ0 (ω1 and θ1) are the 1/e2 beam radius and divergence, respectively, for the incident (diffracted) beam. For a flat-top spectral profile within Δλ, the angular spread for the diffracted output beam 69 (FIG. 1) or 79 (FIG. 2A and FIG. 2B) is increased because of the grating:
                              θ          1                =                                                                              cos                  ⁡                                      (                                          α                      1                                        )                                                                    cos                  ⁡                                      (                                          β                      1                                        )                                                              ⁢                              θ                0                                      +                          Δ              ⁢                                                          ⁢              θ                                =                                                                      cos                  ⁡                                      (                                          α                      1                                        )                                                                    cos                  ⁡                                      (                                          β                      1                                        )                                                              ⁢                              θ                0                                      +                                          g                ⁢                                                                  ⁢                Δλ                                            2                ⁢                                                                  ⁢                                  cos                  ⁡                                      (                                          β                      1                                        )                                                                                                          (        3        )            Considering a more realistic situation where the single-channel output power is distributed within a Gaussian spectral envelope with a 1/e2 width of Δλ, Equation 3 becomes
                              θ          1                =                              θ            0                    ⁢                                    1              +                                                (                                                            g                      ⁢                                                                                          ⁢                      Δλ                                                              2                      ⁢                                                                                          ⁢                                              θ                        0                                            ⁢                                              cos                        ⁡                                                  (                                                      α                            1                                                    )                                                                                                      )                                2                                                                        (        4        )            and M2 for the combined beam is given by
                              M          2                =                              1            +                                          (                                                      g                    ⁢                                                                                  ⁢                    Δλ                                                        2                    ⁢                                          θ                      0                                        ⁢                                          cos                      ⁡                                              (                                                  α                          1                                                )                                                                                            )                            2                                                          (        5        )            At this point, it is useful to note that the peak irradiance on the grating and the combined beam quality are inversely related to ω0. Specifically, using θ0=(λ/πω0)), Equation 5 can be written as:
                              M          2                =                                            1              +                                                (                                                            g                      ⁢                                                                                          ⁢                                              Δλπω                        0                                                                                    2                      ⁢                                                                                          ⁢                                              λcos                        ⁡                                                  (                                                      α                            1                                                    )                                                                                                      )                                2                                              .                                    (        6        )            while for a SBC system with total output power P, the peak irradiance on the grating is given by
                              I          peak                =                              2            ⁢                                                  ⁢            P                                (                                          π                ⁢                                                                  ⁢                                  ω                  2                  2                                                            cos                ⁡                                  (                                      α                    1                                    )                                                      )                                              (        7        )            From the above equations, one sees that for given values of Δλ and g, increasing ω0 decreases Ipeak (i.e., the thermal load on the grating) but simultaneously reduces the combined beam quality. Together Equations 6 and 7 then define a trade space that can be used to determine the required single-channel linewidth for a given combined beam quality and grating peak irradiance goal.
FIG. 3 is a graph that gives the required single-channel linewidth Δλ for a 10-kW, single-grating SBC fiber laser with a combined beam quality M2=1.25. For the plot in FIG. 3, we show the linewidth for Ipeak values of 1.5 kW/cm2 and 6 kW/cm2 versus the grating groove density g. For high-dispersion gratings (>1500 lines/mm) one sees that the linewidth requirement is 10 to 20 pm (10-20 picometers linewidth, which is about 2.5-5 GHz) for even these modest intensities. For fiber lasers where one of the key nonlinearities is stimulated Brillouin scattering (SBS), this relatively large linewidth, compared to the Brillouin linewidth, simplifies high-power scaling.
The broad gain bandwidth of conventional fiber-laser systems allows for operation over a wide range of wavelengths, or even tunable operation. For the simplest fiber-laser system with cavity mirrors having reflectivity across a broad range of wavelengths, the output wavelength can be very broad and can vary with pump power, fiber length, and/or other parameters. The power that can be generated from fiber lasers and fiber-laser amplifiers can often be limited by nonlinear optical effects in the gain and/or delivery fibers used in the system.
It is desirable to produce high peak- and average powers from fiber lasers and amplifiers. Stimulated Brillouin scattering (SBS) and other nonlinear effects such as self-phase modulation (SPM), four-wave mixing (FWM), and stimulated Raman scattering (SRS) are the main effects limiting the output power and pulse energy of a fiber amplifier or laser. To suppress these effects in a fiber amplifier/laser, it is desirable to use a rare-earth-doped (RE-doped) double-clad fiber with a large core. The large core provides two benefits: Spreading the light over a larger core decreases the intensity driving the nonlinear processes, and increasing the core/cladding diameter ratio increases pump absorption, enabling the shortening of the fiber to further reduce nonlinearities. When good beam quality is required, however, increasing the core diameter of the fiber requires that the fiber numerical aperture (NA) be decreased, in order that higher-order modes cannot propagate in the fiber. Using relatively large-core, low-NA fibers with mode-filtering techniques has been demonstrated to achieve good beam quality, but there are practical disadvantages to the use of such fibers. Fibers with very low values of NA exhibit large bending losses, even for relatively large-radius bends. With fibers having the lowest NA, the fiber must be kept quite straight, otherwise the optical amplifier and/or laser has very low efficiency as the bending loss becomes too high. Since a typical laser oscillator or amplifier might require on the order of a meter or more of gain fiber, the inability to coil the fiber has precluded compact packaging of the fiber-laser system.
U.S. Pat. No. 6,324,016 issued to Luster on Nov. 27, 2001 titled TELECENTRIC LENS, and is incorporated herein by reference. Luster described a reflective telecentric lens which uses an on-axis type concave mirror in a pseudo-off-axis manner to avoid blockage of a portion of the field of view. The concave mirror used in a pseudo-off-axis manner permits the telecentric stop, imaging lens, and film or an electronic detector to be moved outside of the field of view.
U.S. Pat. No. 6,822,796 to Takada et al. titled “DIFFRACTIVE OPTICAL ELEMENT” (incorporated herein by reference) describes a method for making blazed gratings having asymmetric grooves with dielectric coatings. U.S. Pat. No. 6,958,859 to Hoose et al. titled “GRATING DEVICE WITH HIGH DIFFRACTION EFFICIENCY” (incorporated herein by reference) describes a method for making blazed gratings having dielectric coatings.
U.S. Pat. No. 5,907,436 titled “MULTILAYER DIELECTRIC DIFFRACTION GRATINGS” issued May 25, 1999 to Perry et al., and is incorporated herein by reference. This patent describes the design and fabrication of dielectric grating structures with high diffraction efficiency. The gratings have a multilayer structure of alternating index dielectric materials, with a grating structure on top of the multilayer, and obtain a diffraction grating of adjustable efficiency, and variable optical bandwidth.
U.S. Pat. No. 7,424,185 titled “STRETCHING AND COMPRESSION OF LASER PULSES BY MEANS OF HIGH EFFICIENCY VOLUME DIFFRACTIVE GRATINGS WITH VARIABLE PERIODS IN PHOTO-THERMO-REFRACTIVE GLASS” issued Sep. 9, 2008 to Glebov et al., and is incorporated herein by reference. This patent describes the design and fabrication of high-efficiency reflective volume Bragg gratings with chirped gratings recorded in photo-thermo-refractive glass having an absolute diffraction efficiency exceeding 95% in transmitting and reflecting modes, which are used to stretch and/or compress ultrashort laser pulses with high efficiency. Glebov et al. describe placement of multiple elements in a compact space, which provides their femtosecond laser system with high efficiency of stretching and re-compression of femtosecond pulses.
There is a need for improved high-power laser systems, particularly fiber-based ring lasers and, in particular, systems that use spectral-beam combining.